The use of supercooled solutions as the source of heat in hot packs is well known. Hot packs are used as infant heel warmers, for the treatment of soreness of muscles, as food warmers, and for other related applications. Certain salts once melted into a solution will remain liquid at a temperature below the melting point of the salt. On solidification, these supercooled solutions give up heat. Typically, water/salt combinations are used to make the supercooled solutions. Other ingredients may be added as well. Commercial solutions generally comprise sodium thiosulfate pentahydrate and dextrose in water, sodium acetate trihydrate in water, and eutectic combinations of sodium thiosulfate pentahydrate and sodium acetate trihydrate. These solutions give up heat when the salt contained therein solidifies or crystallizes.
In order to initiate heat dissemination, the supercooled solution must begin to solidify. Several techniques are known to initiate solidification. These include inserting a crystal of appropriate chemical formulation into the supercooled solution, generating particles within the solution, or generating a localized high energy source within the solution. This latter technique may be accomplished by frictionally generating localized heat within the solution. Inserting a crystal into a supercooled solution, or generating a particle therein, provides a nucleation site at which solidification of the supercooled solution initiates. Accordingly, the crystal or metal particle may be referred to as a nucleating crystal or nucleating particle, respectively. But each of these techniques includes drawbacks.
Inserting a crystal into the supercooled solution requires the use of a separate insertion device. In one form, this insertion device may be a syringe that injects a suspension of crystals into the supercooled solution. In another form, the insertion device may include an attached packet containing a puncture tool. In this latter form, a packet containing a suspension of crystals is intimately attached to the hot pack holding the supercooled solution. Inside this packet is a puncture tool that, when pressed, allows the suspension of crystals to enter the supercooled solution.
Yet another form of this technique places a nucleating crystal between two objects under sufficient pressure to prevent intrusion of the supercooled solution. When the pressure is released, the solution contacts the crystal and solidification is initiated. The syringe device has disadvantages because it is a separate device and the crystals in the suspension can plug the syringe rendering this method inoperative. The device comprising the puncture tool has disadvantages because the packet may be accidentally pressed, such as in packaging or shipping. rendering the hot pack inoperative for its intended purpose. The method using pressure to entrap the crystal is inherently inflexible.
One technique uses metal particles generated from within a supercooled solution by flexing a slotted metal strip contained therein. The metal strip is treated so that rust particles form on its surface. By flexing the metal strip the rust particles may be released into the solution and present a nucleation site. A disadvantage to this technique is the inconsistency of operation. Dependent on the surface preparation, either particles may prematurely initiate nucleation or initiation may not take place. If the surface is too roughened with rust particles nucleation may happen with very little disturbance of that surface resulting in premature nucleation. If the extent of rust formation is too limited, it may be difficult to generate the nucleating particles.
A number of techniques have been developed to generate localized energy sources for the initiation of solidification. Generally, a strip, typically made of metal, and defining holes or slits, is placed into the hot pack containing a supercooled solution. The strip may be pre-formed into a concave-like shape so that when it is depressed and released it springs back to its original shape. Activation occurs with bending or depressing the strip. Drawbacks to this approach include loss of activation ability should the strips break and puncture of the solution container by the broken strips. It has been reported that for some of these devices activation is inconsistent. Another method of generating a localized energy source comprises rubbing together hard objects to generate high localized energy and thus steric hindrance. These hard objects are typically in fluid contact with the supercooled solution. Potentially the hard objects could be rubbed together if the hot pack is mishandled resulting in premature nucleation.
Each of the above methods of initiating nucleation has to some extent proven unreliable or cumbersome. Unreliability of the initiating mechanism diminishes the usefulness of the associated hot pack. What would therefore be advantageous would be to initiate nucleation without the use of a separate insertion device. It would be further advantageous to avoid the inconsistencies of operation of the prior art devices.